When a mobile device is in a geographical location in the home country with no wireless coverage from a home network operator, the mobile device may have no choice but select another network that only provides limited service such as allowing only emergency calls. A cellular wireless network is typically identified by a PLMN (public land mobile network) identifier. Depending on the network coverage, a mobile device can spend a considerable amount of time in limited service state on any other network operator's cell.
For instance, if a mobile device, registered with PLMN-1 in its home country that provides full subscribed services, loses network coverage, it will camp on any other available network in the same country. Due to the lack of roaming agreements between network operators, a mobile device may not be able to obtain its subscribed services on any of these available networks, and will select an “acceptable cell” that belongs to another network operator where it can only make emergency calls. Given the availability of multiple acceptable cells from multiple PLMNs to camp on in limited service state, a mobile device typically selects the first cell that it failed to register on, regardless of PLMN configuration.
The home network of a roaming user (e.g. Country: A, PLMN-1) may have similar roaming agreements with multiple network operators in another country (e.g. Country: B, PLMN-2 and PLMN-3). The mobile device may have multiple options for selecting a network to roam on in such a situation. This may, for example, be the case when no information about roaming networks is specified on the subscription module of the device, for example in a “PLMN Selector” List, “User Controlled PLMN Selector with Access Technology” list or an “Operator Controlled PLMN Selector with Access Technology” list in a Universal Subscriber Identity Module (USIM) or similar module that resides on the device. Examples of such modules include (but are not limited to) Subscriber Identity Module (SIM), IP Multimedia Services Identity Module (ISIM), and Removable User Identity Module (RUIM), CDMA SIM (CSIM). It may also be the case that no network specified in the USIM or similar module is available at the location. In such circumstances, the mobile device may select and attempt registration on another available PLMN. There may be limitations placed on the selection of another PLMN. For example, the technical specification (TS) of 3rd Generation Partnership Project (3GPP) states that any PLMN that offers a “high quality” signal can be selected, where “high quality” is as defined in TS 3GPP 23.122 and TS 25.304.